THORN 3500 CHASSIS

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The set has been reconverged, but when the picture content
is dark the colours go out of convergence and there’s
pincushion distortion at the top of the raster. There’s also a
very intermittent fault, the picture and sound going off for
about a second, accompanied by an h.t. flash noise. The
fault occurs once every four months or so.
The convergence and geometry variations with
brightness could be due to low e.h.t. (possibly a faulty
tripler) or a low -emission c.r.t. The intermittent loss of
sound and picture will probably be cleared by replacing the
following: the chopper driver transistor VT602, W609 (use
two 1N4002 diodes in parallel) which is in series with the
chopper transistor, the 30V supply reservoir capacitor
C607 (1,000µF, 64V) and the 30V supply reference diode
W605.

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